It is known that, when planning suspension bridges one must forcedly take into account the action of the wind, which not only involves a substantially static cross-wind pressure, but also vibrations. Two distinct planning theories are followed at present:
on the one hand, according to a more ancient technique, the bridge framework is formed of a trestle structure which is particularly rigid and of considerable height compared to the width of the bridge; this type of structure opposes an essentially static resistance to the aerodynamic wind forces; PA1 on the other hand, according to a more modern technique, the bridge framework is formed of a more slender and thus more flexible structure, which is however adapted to oppose an essentially dynamic resistance to the action of the wind.
As known, these suspension bridges have vibration frequencies of their own; normally, with no wind, the basic flexural vibration frequency differs from the basic torsional vibration frequency, both being generally very low. Nevertheless, the action of side winds varies said typical vibration frequencies, particularly because--especially in bridges with large transversal dimensions and/or a wide span, for instance motorway bridges--the flat suspended structure behaves, when actually exposed to side winds, similarly to a wing surface, hence with a "lifting" effect which greatly varies from one moment to the next.
As wind increases its force, the two aforespecified vibration frequencies tend to approach, up to the point of coinciding: in these circumstances, the structure is thus subjected to so-called "flutter" conditions, i.e. to flexural-torsional stresses which may even be dangerous for the stability of the whole bridge framework.
A suspension bridge of this second type--and the problems connected therewith, particularly those concerning the flutter phenomena--is described in EP-A-0.233.528, filed by the same Applicant, to which reference is made for a better understanding of the present invention.
The main object of EP-A-0.233.528 is a wing structure, which is rigidly fixed to the lateral edges of the bridge framework and is meant to increase the flutter speed of the bridge beyond the top speed of the wind expected in the bridge area. Nevertheless, said wing structure is conceived so as to be substantially independent from the framework; in fact, the EP-A-0.233.528 by no means describes said framework.
Studies carried out by the Applicant on these types of suspension bridges, including tests in a wind tunnel, have however proved how also the structure of the framework is of considerable importance for the behaviour of the bridge in strong wind conditions.